The long-term aim of this proposal is to understand the neural control of the human postural motor system. The applicants will use kinesiology (EMG, digital kinematic recording, and forceplate pressure) following a toe-up/down platform perturbation in patients with postural instability due to specific age- related neuro-degenerative diseases. Parkinson's patients showed enlarged medium latency responses. reversed distal-proximal sequencing patterns, impairment of EMG response amplitude scaling during predictable perturbations and inability to modify their stabilizing response by cognitive set during random perturbations. From this, the applicants hypothesize that: (1) cognitive and attentional functions of association cortex are important in postural stability; and (2) those functions are separate from scaling function of the basal ganglia. To test this hypothesis, they will perform postural reflex studies on: (1) typical Alzheimer's patients with progressive memory and learning impairment; (2) Alzheimer's patients with extrapyramidal features; (3) Parkinson's patients with and without frontal lobe abnormalities; and (4) elderly age/sex-matched normals. The patients will also receive extensive behavioral and SPECT studies which will define the cognitive and postural sets that can separate cognitive from motor performance factors in postural stability. The correlation of specific functional deficits with the degree of frontal cortex involvement will show the relative influence of association cortex and basal ganglia contributions to postural regulation. The results of this study should add understanding of postural instability in Alzheimer's and other patients and will assist in detecting patients at risk for falls and help in prevention and rehabilitation. It may also potentially provide an objective means of monitoring future treatment in patients with Alzheimer's disease.